The term “to crimp” is internationally established and defined in standards. In practice, however, terms such as “press”, “squeeze”, “stamp”, or “apply” are also used. “Crimping” is to be understood as the creation of an unreleasable electrical and mechanical connection between a conductor and a contact. In the crimping operation, the material of the crimp contact and of the conductor that are to be connected is plastically, permanently deformed. When this occurs, poorly conducting surface layers, if present, are broken open, which favors the electrical conductivities. A correct crimping also prevents the penetration of corrosive media even under difficult operating conditions such as change of temperature or vibration.
The goal of crimping is to create a good mechanical and electrical connection which remains qualitatively unchanged in the long term.
For the purpose of crimping, use is made of contact-specific crimping tools with a stationary crimping anvil below and a vertically displaceable crimping stamp above (see FIG. 1 to FIG. 3). Mounted in the crimping tool are the crimping stamp for the conductor crimp and the crimping stamp for the insulation crimp, which usually by means of notched disks with different height cams can be set independently of each other in the vertical direction to the conductor diameter or the insulation diameter respectively. These settings directly affect the quality of the crimped connection.
In the case of open crimp contacts (see FIG. 4 and FIG. 5), feeding of the wire takes place above the contact. Usually, the previously insulation-stripped conductor is correctly positioned by machines for the crimping operation simultaneously in the radial and the axial direction. Through the downward movement of the crimping stamp, first the conductor is lowered by means of a mechanism into the upwardly opened conductor and insulation-crimp claws, after which the crimping operation per se begins, with deformation of the lugs corresponding to the forms of the crimping stamps. After the stroke of the crimping stamp, the crimp has the desired pressed form (see FIG. 5), which again depends on the contact sheet metal that is used, the conductor cross-section, the copper of the conductor, and the insulation stripping. With closed contacts, after being radially aligned the conductor must be inserted axially into the crimping area of the contact, which has the shape of a tube.
A cross-sectional view of a faultlessly executed crimped contact shows the originally individual round strands of the conductor pressed compactly against each other into polygons. In the crimp area of the contact, the inner surface shows deformations of the contact points of the individual strands.
An important parameter for the degree of pressing of the conductor crimp is the Crimp Compression Ratio (OCR), defined as the ratio of the cross-sectional area of the crimped conductor crimp (CCS), see FIG. 6a, to the sum of the cross-sectional areas of the conductor (WCS) and of the contact part (TCS) before deformation, see FIG. 6b, according to the formula:
      C    ⁢                  ⁢    C    ⁢                  ⁢    R    =                              C          ⁢                                          ⁢          C          ⁢                                          ⁢          S                                      W            ⁢                                                  ⁢            C            ⁢                                                  ⁢            S                    +                      T            ⁢                                                  ⁢            C            ⁢                                                  ⁢            S                              ·      100        ⁢    %  
A quality goal is to attain a certain Crimp Compression Ratio (CCR) irrespective of whichever conductor cross-section is processed. This is achieved by the corresponding crimp height being specified for each conductor cross-section.
The conductor crimp must enclose all of the individual strands. At the front end of the conductor crimp the individual strands, depending on their cross section, must project by about 0.5 mm and must not disappear into the crimp. In the window that is situated between the conductor crimp and the insulation crimp, the conductor and the conductor insulation must be visible. The insulating crimp must surround the insulation without penetrating into the latter.
Important criteria for the appraisal of a crimped connection are the crimp form, the crimp height as a measure of the Crimp Compression Ratio, and the conductor pull-out strength. However, these criteria are only suitable when setting up the crimping machine and for random sampling during production. To satisfy the present-day quality requirements for all crimped connections, means must be available which, during the crimping operation, can record, analyze, and save crimping data about each crimped connection and influence results-oriented machine data. For the appraisal of the crimped connection (without mechanical destruction of the crimped connection) the crimping force is placed in relation to the crimping distance or the crimping time. With corresponding analysis of the crimping data, the quality of a crimped connection can be reliably appraised.
A method or device for appraisal of the quality of a crimped connection must detect crimp faults such as incorrect insulation crimp height, incorrect conductor crimp height, omitted strands in the conductor crimp, incorrect or no stripped insulation length, incorrect insertion depth, or strands cut off during insulation-stripping, and generate corresponding error messages.
Prior Art
From European patent application EP 0 460 441 a method has become known for the detection of missing strands, or of crimped-in conductor insulation, in a crimped connection by reference to the pattern of the crimping force. During a crimping operation, value pairs consisting of crimping force and position of the crimping stamp are measured and saved. The value pairs that are measured during the creation of a crimped connection give the pattern of crimping force of the crimping operation with the crimping force depending on the position of the crimping stamp. The section of the curve with sharply increasing force is linearized and a point is determined from the mean of the minimum and the maximum crimping force. The point is compared with a reference value. If the point lies within a predefined deviation from the reference value, the crimped connection is of acceptable quality. When analyzing the pattern of the crimping force of the crimping operation, the maximum crimping force is also considered. If the maximum crimping force deviates excessively from a reference value, the crimped connection is rejected as unusable. The point in the section of the curve with sharply increasing force, and the maximum crimping force, provide information about missing strands or about crimped-in conductor insulation in the crimped connection.
In a normal commercially available crimping press, during the crimping operation a force sensor registers the force, which is saved in digital form as a force-dependent curve pattern. This is compared with a reference curve. Depending on the magnitude of the deviation from the reference, the type of crimping fault is determined.
Disadvantageous with this method is that, despite great outlay for computing, saving, and calculation, no meaningful statement about the quality of the crimped connection is possible.
Also known from prior art European EP 0 902 509 B1 is a crimping device having a crimping stamp with which a contact can be connected with a conductor. The crimping device includes a force sensor which is arranged above the crimping stamp to determine the crimping force.
To determine the quality of the crimped connection, the crimping force curve is plotted and subdivided into several zones. To determine the width of the first and of the second zone, the width of the fourth zone is multiplied by a factor between 0 and 2. The highest point on the reference crimping force curve is normalized to 100%. The width of the third zone is then determined through the two 90% points on the reference crimping force curve.